The present invention is directed to implantable cardiac electrical stimulation devices. More specifically, the present invention is directed to a cardiac electrical stimulation device possessing automatic capture with a high threshold response and patient notification method.
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacing and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as xe2x80x9ccapture.xe2x80x9d In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
The capture xe2x80x9cthresholdxe2x80x9d is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Modern pacemakers have incorporated techniques for monitoring the cardiac activity following delivery of a stimulation pulse in order to verify that capture has indeed occurred. If a loss of capture is detected by such xe2x80x9ccapture-verificationxe2x80x9d algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse energy. This approach, called xe2x80x9cautomatic capturexe2x80x9d improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient""s heart has been effective, and 2) greatly increasing the device""s battery longevity by conserving the energy used to generate stimulation pulses.
Commonly implemented techniques for verifying capture involve monitoring the internal myocardial electrogram (EGM) signals received on the implanted cardiac electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an xe2x80x9cevoked,xe2x80x9d which is the intracardiac P-wave of R-wave that indicates depolarization of the respective cardiac tissue, may be detected. The depolarization of the heart tissue in response to the heart""s natural pacing function is referred to as xe2x80x9cintrinsic responsexe2x80x9d. Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by ventricular sensing circuits of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles. If no evoked response is detected, typically a higher-energy, back-up stimulation pulse is delivered to the heart very shortly after the primary ineffective stimulus, commonly in the order of 60-100 ms, in order to maintain the desired heart rate.
The output of the primary pulse is then progressively increased to restore stable capture. This is followed by an automatic threshold test to determine the minimum pulse required to capture the heart at that time. Threshold tests may also be performed on a periodic basis, for example three times a day, daily or weekly. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter, the output level is progressively decremented until capture is lost after which the output is progressively increased in small steps until capture is reestablished. The stimulation pulse energy is then set to a level above the lowest output level at which capture was attained. This additional working margin above the measured threshold allows for small fluctuations in threshold to occur without risk of loss of capture and frequent delivery of both back-up pulses and initiation of the threshold testing sequence. A safety margin for the patient is provided by a fixed significantly higher output of the back-up pulse. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Such automatic methods for verifying and maintaining capture are currently implemented in cardiac stimulation devices utilizing bipolar sensing and unipolar stimulation. In commercially available cardiac pacing devices with automatic capture verification capabilities, a fixed maximum stimulation pulse energy is set, at which the autocapture feature becomes disabled. The advantage of setting a maximum stimulation pulse energy limit is to minimize patient discomfort should the output be increased to the maximum allowed setting in the situation of rising thresholds. This maximum value is higher than the default output value for most pacemakers recently introduced to the market.
Automatic capture routines thus improve pacemaker performance as long as the capture threshold remains within a normal range of stimulation pulse energies. However, if a pacing-dependent patient undergoes an unexpected, massive increase in threshold, for example if an electrode shifts acutely or the threshold rises on a chronic basis due to progression of disease or as a side-effect of a new pharmacologic agent, the fixed maximum stimulation pulse energy allowed by Autocapture may not effectively capture the heart. Therefore, in these cases, further increases in pulse energy may be needed.
Allowance of high-energy output will afford the patient greater protection against ineffective stimulation. Therefore, it would be desirable to allow exceptional conditions to supercede the present autocapture maximum output limit. With automatic capture enabled, the frequency of follow-up evaluations may have been reduced, but the patient will be protected by the automatic capture algorithm in the presence of a rising capture threshold.
However, a rising output energy requirement could deplete the pacemaker battery energy more quickly than under normal stimulation conditions resulting in device failure prior to the next scheduled follow-up evaluation. Notifying the patient that a change in stimulation conditions has occurred that warrants medical evaluation would therefore be important to not only prevent unexpected battery depletion but allow further evaluation as to the reasons for the significant rise in the capture threshold.
Patient warning systems have been proposed to alert patients of conditions that warrant medical attention such as a detected lead failure, impending battery depletion, or loss of capture. Reference is made to U.S. Pat. No. 5,076,272 to Ferek-Petric and U.S. Pat. No. 4,140,131 to Dutcher et al. In typical patient warning systems, detection circuitry for identifying conditions that warrant patient notification or warning are included, such as lead impedance measurements for detection of lead failure or battery voltage level detectors. Not addressed heretofore is the recognition of an appropriate increase in stimulation output in response to a large increase in capture threshold, which may accelerate battery depletion or signal either a potential problem with the pacing lead or a change in the patient""s clinical status that would warrant further evaluation.
Typical patient warning systems involve additional hardware or circuitry. Additional electrodes or conductive elements implanted away from the heart have been proposed to provide muscle stimulation (or nerve stimulation) to alert a patient of a condition that warrants medical attention. Reference is made to the U.S. Pat. No. 5,076,272 to Ferek-Petric; U.S. Pat. No. 4,140,131 to Dutcher et al. supra; U.S. Pat. No. 5,549,653 to Stotts et al.; U.S. Pat. No. 5,609,615 to Sanders; and U.S. Pat. No. 5,643,328 to Cooke et al.
With additional dedicated electrodes or conductive elements for patient warning stimulation, dedicated output circuitry is required to deliver stimulation pulses to the patient warning element, or additional circuitry is required to divert stimulation output from standard heart leads to the patient warning element. Implementation methods for delivering stimulation to an auxiliary stimulation element for patient warning is described U.S. Pat. No. 5,076,272 to Ferek-Petric; U.S. Pat. No. 4,140,131 to Dutcher et al. supra. Reference is also made to U.S. Pat. No. 5,709,712 to Paul et al., and U.S. Pat. No. 4,407,288 to Langer et al. Another approach for delivering patient warning stimulation is the use of one output channel of a dual chamber cardiac stimulation device. By placing a pin electrode in one output channel, stimulation of the local tissue can be achieved, however, the dual chamber stimulation device is functionally reduced to a single chamber stimulation device. Reference is made to U.S. Pat. No. 5,549,653 to Stotts et al.
It would therefore be desirable to provide, in a cardiac stimulation device possessing automatic capture verification and maintenance methods, a method for responding to a high increase in capture threshold in order to provide further assurance of effective stimulation therapy. In case high-energy stimulation is required, it would also be desirable to provide a method for notifying the patient that a change in stimulation conditions has occurred and medical attention should be sought. It would be further desirable to provide a patient warning system without requiring additional circuitry or hardware or otherwise limiting the cardiac stimulation device functionality.
The present invention addresses these needs by providing a stimulation device and method for providing automatic capture verification and threshold testing capabilities with an added high-threshold response algorithm and patient notification method. To this end, the stimulation device responds to higher than normal capture thresholds by permitting the stimulation pulse energy to be set above the fixed maximum pulse energy normally allowed by automatic capture verification techniques.
One object of the present invention is to provide a high threshold response algorithm that sets the pulse energy to the maximum device output after repeated automatic threshold tests result in the identification of a high threshold. In an alternative embodiment, the pulse energy may be incremented in a step-wise fashion, above the maximum pulse energy normally allowed by an automatic capture verification routine. In this embodiment, periodic threshold tests are repeated to ensure the incremented pulse energy remains above threshold. In order to maintain comfortable yet effective stimulation, the high-energy stimulation is delivered in a bipolar configuration, rather than unipolar, to avoid patient discomfort. Automatic capture detection that normally requires unipolar stimulation is disabled.
Another object of the present invention is to alert the patient of the change to high-energy output when it occurs. After the high-energy output is automatically set, one or more high-energy stimulation pulses are delivered in a unipolar configuration on a scheduled or event-triggered basis. A unipolar high-energy stimulation pulse is typically perceptible by the patient due to stimulation of excitable tissue surrounding the device housing. Sensation of a periodic high-energy pulse alerts the patient that stimulation conditions have changed and medical attention should be sought.
Thus, one feature of the present invention is a method that responds to a large increase in capture threshold by automatically setting a high-energy stimulation pulse output. Another feature of the present invention is the ability to switch stimulation electrode configuration from unipolar to bipolar stimulation when the high-energy output is set in response to a high threshold. Yet a further feature of the present invention is a method for notifying the patient that a significant change in stimulation condition has occurred.
The foregoing and other objects and features of the present invention are realized by providing an implantable stimulation device equipped with cardiac data acquisition capabilities. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device and executing various test algorithms such as capture verification and threshold testing; a set of leads for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; switching circuitry to allow switchable selection of sensing and stimulation electrode configurations; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses. In addition, the device includes memory for storing operational parameters used by the control system. The device also includes a telemetry circuit for communicating with an external device.
When operating according to a preferred embodiment, the control system performs a threshold search whenever a sustained loss of capture is detected, e.g. loss of capture on at least two consecutive cycles, according to known automatic capture techniques. If the threshold test results for two consecutive threshold tests indicate a large increase in threshold, automatic capture is disabled by the control system and the stimulation pulse energy is reset to the highest available output. Alternatively, the pulse energy is increased in a step-wise fashion. The electrode configuration is switched to bipolar by switching circuitry under control of the control system. A patient notification to the changed stimulation conditions is delivered by switching the electrode configuration back to unipolar for one or more stimulation pulses after which bipolar stimulation is again resumed. The patient notification may occur at a scheduled time of day, or upon a pre-defined event such as a given number of stimulation pulses or stimulation at the programmed rest rate.
In an alternative embodiment, a tiered response to rising capture threshold is provided. The stimulation output is adjusted to a level equal to the capture threshold plus a working margin whenever a threshold test is performed. The working margin applied depends on the range of output settings in which the capture threshold falls. For relatively low capture thresholds, a normal working margin is applied. For relatively high capture thresholds a larger working margin is applied. If the stimulation output exceeds a predefined alarm level, the patient notification system is enabled.
In another embodiment, the high threshold response includes lead surveillance. Lead impedance measurements are made to determine if lead failure is the likely cause of the large increase in capture threshold. If so, the high output stimulation is delivered using a unipolar electrode stimulation configuration to eliminate the use of the defective lead.
The system and method of the present invention thus provide an appropriate response to a massive rise in capture threshold by allowing the stimulation pulse energy to be set to a high output as necessary to maintain effective stimulation therapy. The present invention further provides a patient warning system to alert the patient of this change in stimulation conditions so that medical attention may be sought. The rise in threshold may indicate a change in clinical condition that warrants medical attention. The increased output will drain battery energy more quickly than anticipated. By making a clinician aware of the altered stimulation conditions, patient follow-up can be scheduled as needed. The present invention thus improves stimulation device performance by ensuring safe and effective stimulation therapy even under high threshold conditions without risking unexpected battery depletion.